As methods for achieving high-degree QoS (Quality of Service) functions, radio transmission devices, accompanied with adaptive modulation functions for switching over system gains and transmission capacities in response to fluctuations of quality of radio transmission lines, may employ a method for installing QoS functions in radio transmission devices and a method for combining radio transmission devices with MAC frame transmission devices serving as Layer-2 switches (L2SW).
In general, radio transmission devices serving as transmission sources conduct communications with their target transmission destinations via a plurality of other radio transmission devices. Installing QoS functions in all radio transmission devices relaying communications may cause unnecessary multiple functions and useless installation cost; hence, Qos functions are limitedly installed in specific stations needing QoS functions such as stations connected with external devices and nodes integrating a plurality of lines. Additionally, QoS functions may be installed in common radio transmission devices between relay stations that relay communications in combination with external L2SW.
Radio transmission capacities, which are narrower in frequency range than wired networks, need QoS functions to transfer MAC frames (i.e. frames recording MAC protocols) residing in radio transmission devices in accordance with priorities.
To dynamically change transmission capacities in radio transmission devices, adaptive modulation functions need to dynamically change Qos settings. In the conventional method for combining the existing L2SW with radio transmission devices, the L2SW is unable to detect fluctuations of radio transmission capacities; hence, it is impossible to perform QoS control in response to radio transmission capacities.
Next, the conventional constitution of L2SW will be described with reference to FIG. 6.
This L2SW performs interstation flow control using PAUSE frames based on IEEE802.3. According to the flow control prescribed by IEEE802.3, when a used quantity of a reception buffer exceeds a predetermined value which is determined in advance, PAUSE frames multiplying PAUSE times are transmitted to line-opposing stations to stop transmission with line-opposing stations, whereas when a used quantity of a reception buffer becomes lower than the predetermined value, PAUSE frames multiplying a PAUSE time “0” are transmitted to line-opposing stations so as to restart transmission with line-opposing stations. In this case, all frames are collectively stopped in transmitting irrespective of priorities and band controls of frames so that high-priority frames are stopped in transmitting; hence, it is difficult to secure adequate QoS control.
Next, the operation of the L2SW (A1) shown in FIG. 6 will be described with reference to a flowchart of FIG. 7.
In the L2SW (A1), a CPU (A2) extracts a PAUSE frame (A8) from a reception traffic (A7) so as to input a reception signal (A81) and forward it to a scheduler (A3) as a PAUSE detection signal (A21). According to a control flow shown in FIG. 7, the scheduler (A3) controls three priority buffers (A4, A5, A6), corresponding high/middle/low priorities, by use of priority flow control signals (A31, A32) based on the PAUSE detection signal (A21).
The flow control logic of the scheduler (A3) will be described with reference to the flowchart of FIG. 7.
Upon receiving the PAUSE detection signal (A21) from the CPU (A2) (step SA1), the scheduler (A3) of the L2SW (A1) makes a decision as to whether or not PAUSE control is currently in progress (step SA2), wherein the scheduler (A3) stops transmission using the low priority buffer (A6) if PAUSE control is not currently in progress (step SA3). Additionally, the scheduler (A3) stops transmission using the middle priority buffer (A5) while PAUSE control is in progress (step SA6).
When the scheduler (A3) receives a PAUSE release frame, or when a PAUSE control time (i.e. a transmission stop time) overpasses a prescribed time specified by a PAUSE time field of a PAUSE frame (step SA4), the scheduler (A3) releases stoppage of transmission so as to start transmission using all of the high/middle/low priority buffers (A4, A5, AG) (step SA5). Regarding this, when the above condition is not established, the scheduler (A3) continues stoppage of transmission using the corresponding priority buffer.
When the L2SW (A1) is combined with a radio transmission device having an adaptive modulation function as described above, the radio transmission device undergoes a reduction of its radio transmission capacity due to adaptive modulation. When congestion occurs in radio-direction traffic, the radio transmission device outputs a PAUSE frame to the L2SW (A1), which in turn stops transmission of the low priority buffer (A6). Upon receiving a PAUSE frame again during PAUSE control in progress, the L2SW (A1) stops transmission of the middle priority buffer (A5). Thus, it is possible to achieve flow control specified for each priority in response to the number of reception times of PAUSE frames.
Next, the conventional constitution of another L2SW will be described with reference to FIG. 8. FIG. 9 is a flowchart showing the operation of an L2SW (B1) shown in FIG. 8.
In the L2SW (B1), a shaper (B3) executes shaping on a transmission buffer (B4) based on a PAUSE detection signal (B21) from a CPU (B2) in accordance with the control flow of FIG. 9, thus adjusting a transmission rate of a transmission signal (B41).
Next, the control logic of the shaper (B3) will be described with reference to the flowchart of FIG. 9.
Upon detecting the PAUSE frame (step SB1), the shaper (B3) starts shaping to decrease a transmission rate of the transmission buffer (B4) (step SB2). Upon receiving a PAUSE release frame, or upon expiration of a prescribed time specified in a PAUSE time field of a PAUSE frame (step SB3), the shaper (B3) stops shaping (step SB4) so as to restart data transmission at the original transmission rate (step SB5). Upon receiving a PAUSE frame again during PAUSE control in progress, the shaper (B3) further decreases its transmission rate so as to receive a PAUSE release frame or repeat the above operation until the PAUSE time expires.
When the L2SW (B1) is combined with a radio transmission device having an adaptive modulation function as described above, it is possible to control a transmission rate via a PAUSE frame, wherein the radio transmission device undergoes a reduction of its radio transmission capacity due to adaptive modulation. When congestion occurs in radio-direction traffic, a PAUSE frame is transmitted to the L2SW (B1), which in turn decreases its transmission rate in response to the number of reception times of PAUSE frames; this makes it possible to prevent congestion of radio-direction traffic.
In the constitution shown in FIGS. 6 and 8 in which the existing L2SW is combined with a radio transmission device, the L2SW is able to detect a congested state of the radio transmission device based on the number of reception times of PAUSE frame alone; hence, the L2SW is able to notify simple information. The L2SW (A1) shown in FIG. 6 performs flow control per each priority whilst the L2SW (B1) shown in FIG. 8 updates its transmission rate; hence, they are each able to control a single item. With a simple flow control per each priority, it is impossible to implement QoS control for securing an appropriate ratio of transmission frames per each priority.
Since the existing L2SW starts its control by receiving a plurality of PAUSE frames, the L2SW involves a latency between the timing of fluctuations of radio transmission capacities and the timing of updating its transmission rate or the timing of executing flow control; this may be an obstacle in stabilizing the quality of radio transmission lines. Additionally, the existing L2SW control is unable to keep track of continuous fluctuations of radio transmission capacities.
It is necessary to install a relatively large-capacity buffer in each radio transmission device in order to continuously receive frames without exception during the latency until detection of a congested state of the L2SW switch in its radio-direction traffic. Installing a large-capacity buffer in each radio transmission device is comparable to installing a QoS function individually in each radio transmission device because it provides relatively few functions and increases installation cost; hence, it is difficult to practically achieve.
In general, the signal transmission quality of a radio communication system depends upon conditions of radio transmission lines; hence, a normal reception power during operation is set to be higher than a prescribed reception power for securing the minimum quality of communication. During normal operation, radio transmission lines may undergo a reduced resistance to fluctuations, but transmission capacities can be increased by adopting a multi-valued modulation method. Irrespective of low quality radio transmission lines, it is possible to prevent an instantaneous power failure of radio transmission lines by adopting a modulation method (with a small multi-value number) demonstrating a high resistance to fluctuations of radio transmission lines however, which in turn entails a reduction of transmission capacities. That is, when a modulation method is changed in response to conditions of radio transmission lines, it is possible to maximize radio transmission capacities, and to thereby secure minimum quantity of radio transmission capacities. This is called an adaptive modulation method, for example, which is disclosed in Patent Document 1.
In general, radio transmission capacities are smaller than frequency ranges of wired networks, and radio communication devices may accumulate frames in buffers with respect to burst signals transmitted with LANs (Local Area Networks); hence, it is necessary to perform QoS control for transmitting MAC frames in an order of higher-priority ones in radio transmission.
Patent Document 2 and Patent Document 3 disclose technologies using PAUSE frames based on IEEE802.3, wherein they perform flow control and communication band adjustment not in units of ports but per each priority.